Directional cell migration towards extracellular signals is essential for normal tissue morphogenesis and repair, and abnormalities in the regulatory pathways involved result in severe pathological consequences during tumor metastasis. This proposal focuses on the role of the adenomatous polyposis coli (ARC) multi- protein complex in regulating microtubule and actin cytoskeletons in response to extracellular signals that stimulate cell migration and adhesion. APC was originally discovered as a tumor suppressor protein that controls the level of beta-catenin. APC is a large scaffolding protein with many binding partners that also regulates microtubules and directional cell migration in response to extracellular signals. Although the importance of APC in diverse morphological processes is well established, very little is known about how extracellular signals affect its interaction with cytoskeletal binding partners and regulation of microtubule dynamics, or how the APC scaffolding complex influences membrane dynamics and cell migration Our hypothesis is that APC coordinates microtubule and actin reorganization by acting as scaffold for cytoskeletal regulators, and that in direct response to extracellular signaling components of the APC scaffold are locally recruited and activated to specify directional membrane extension. We propose a rigorous biochemical analysis of APC regulation and functions combined with novel assays to reconstitute APC- microtubule dynamics in vitro and to image with high spatiotemporal resolution APC-microtubule dynamics in cells. We propose to: 1). Define mechanisms involved in extracellular signal-induced modifications of APC multi-protein complex composition and functions; 2). Dissect and reconstitute effects of APC multi-protein complex components on microtubule dynamics and F-actin binding in vitro; and 3). Characterize APC- mediated localized changes in microtubule and F-actin dynamics in response to extracellular signals. The significance of these proposed studies is that they will define protein-protein interactions in the APC complex that locally regulate cytoskeleton reorganization during cell extension and contact formation, and how the function of this complex is regulated by signaling pathways important in development and disease.